In a case where the electron optical system of an electron microscope is intended to be automatically adjusted, it is not easy to carry out automation similar to the automatic focusing function of general purpose optical cameras, video cameras or the like. The main reasons for this include (1) problems on S/N ratio and (2) aberration characteristics.
The S/N ratio of electron microscope images is low unlike general images, so that the processing of the differential system of picture signals does not function. If an image having a high S/N ratio is intended to be obtained, the dose of electron beams must be increased. However, the irradiation damage of a sample causes a problem, so that the adjusting method for acquiring many images further increases this problem. In addition, a high resolution must be achieved. Finally, even if the S/N ratio is low, the processing of the differential system of picture signals must function. This makes the automatic adjusting method difficult.
The specific aberration characteristics of electron microscopes also make the adjusting method difficult. In general purpose optical cameras, only the focus may be adjusted. However, in the optical systems of electron microscopes, astigmatism correction must be also adjusted in addition thereto. Since there are two adjusting quantities (with respect to x and y directions) in the adjustment of astigmatism correction, the adjusting method is further complicated, so that it is difficult to carry out the adjusting method. In addition, unlike optical lens systems, spherical aberration can not be corrected except for very special cases. Particularly in high resolution electron microscopes, optimum conditions can not be obtained on a so-called Gaussian plane. This also makes the adjusting method difficult.
Thus, the electron optical system can not easily adjusted, so that it is desired to automate the adjustment of the electron optical system.
In conventional three-dimensional electron microscopes, it is required to acquire many images in order to automatically adjust the electron optical system. Naturally, it is required to obtain precise series images under the same optimum optical conditions, so that there is a problem in that this is complicated and is not easily achieved.
Moreover, when obtained electron microscope images are utilized for carrying out focusing in conventional electron microscopes, electron microscope images are naturally reflected in characteristics of samples, such as compositions and shapes, so that it is not possible to recognize electron optical states from which characteristics of the samples are abstracted. For that reason, there is a problem in that it is not possible to precisely carry out focusing since the degree of focusing depends on characteristics themselves of the samples.
In addition, if astigmatism exists, it is required to correct astigmatism by a so-called stigmater while it is not possible to precisely carry out focusing.